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Symptoms

A BitLocker-protected computer may be vulnerable to Direct Memory Access (DMA) attacks when the computer is turned on or is in the Standby power state. This includes when the desktop is locked.

BitLocker with TPM-only authentication lets a computer to enter the power-on state without any pre-boot authentication. Therefore, an attacker may be able to perform DMA attacks.

In these configurations, an attacker may be able to search for BitLocker encryption keys in system memory by spoofing the SBP-2 hardware ID by using an attacking device that is plugged into a 1394 port. Alternatively, an active Thunderbolt port also provides access to system memory to perform an attack. Note that Thunderbolt 3 on the new USB Type-C connector includes new security features which can be configured to protect against this type of attack.

This article applies to the following systems:

Systems that are left turned on

Systems that are left in the Standby power state

Systems that use the TPM-only BitLocker protector

Cause

1394 physical DMA

Industry standard 1394 controllers (OHCI compliant) provide functionality that allows for access to system memory. This functionality is provided as a performance improvement. It enables large amounts of data to transfer directly between a 1394 device and system memory, bypassing CPU and software. By default, 1394 Physical DMA is disabled in all versions of Windows. The following options are available to enable 1394 Physical DMA:

An administrator enables 1394 Kernel Debugging.

Someone who has physical access to the computer connects a 1394 storage device that complies with the SBP-2 specification.

1394 DMA threats to BitLocker

BitLocker system integrity checks protect against unauthorized Kernel Debugging status changes. However, an attacker could connect an attacking device to a 1394 port, and then spoof an SBP-2 hardware ID. When Windows detects an SBP-2 hardware ID, it loads the SBP-2 driver (sbp2port.sys), and then instructs the driver to allow for the SBP-2 device to perform DMA. This enables an attacker to gain access to system memory and search for BitLocker encryption keys.

Thunderbolt physical DMA

Thunderbolt is a new external bus that has functionality that allows for direct access to system memory. This functionality is provided as a performance improvement. It enables large amounts of data to transfer directly between a Thunderbolt device and system memory, thereby bypassing the CPU and software. Thunderbolt is not supported in any version of Windows, but manufacturers might still decide to include this port type.

Thunderbolt threats to BitLocker

An attacker could connect a special purpose device to a Thunderbolt port and have full direct memory access through the PCI Express bus. This could enable an attacker to gain access to system memory and search for BitLocker encryption keys. Note that Thunderbolt 3 on the new USB Type-C connector includes new security features which can be configured to protect against this type of access.

Resolution

Some configurations of BitLocker can reduce the risk of this kind of attack. The TPM+PIN, TPM+USB, and TPM+PIN+USB protectors reduce the effect of DMA attacks when computers do not use sleep mode (suspend to RAM). If your organization allows for TPM-only protectors or supports computers in sleep mode, we recommend that you block the Windows SBP-2 driver and all Thunderbolt controllers to reduce the risk of DMA attacks.

For more information about how to do this, go to the following Microsoft website:

SBP-2 Mitigation

On the previously mentioned website, refer to the "Prevent installation of drivers matching these device setup classes" section under "Group Policy Settings for Device Installation".

The following is the Plug and Play device setup class GUID for an SBP-2 drive:

d48179be-ec20-11d1-b6b8-00c04fa372a7

Thunderbolt Mitigation

Important The following Thunderbolt mitigation only applies to Windows 8 and to Windows Server 2012. It does not apply to any of the other operating systems that are mentioned in the "Applies to" section.

On the previously mentioned website, refer to the "Prevent installation of devices that match these device IDs" section under "Group Policy Settings for Device Installation".

The following is the Plug and Play compatible ID for a Thunderbolt controller:

PCI\CC_0C0A

Notes

The drawback of this mitigation is that external storage devices can no longer connect by using the 1394 port, and all PCI Express devices that are connected to the Thunderbolt port will not work. Because USB and eSATA are so prevalent, and because DisplayPort often works even when Thunderbolt is disabled, the adverse effect caused by these mitigations should be limited.

If your hardware deviates from current Windows Engineering Guidance, it may enable DMA on these ports after you start the computer and before Windows takes control of the hardware. This opens your system to compromise, and this condition is not mitigated by this workaround.

More Information

For more information about DMA threats to BitLocker, see the following Microsoft Security blog: